- Astronomy & Astrophysics

Astronomy
&
Astrophysics
A&A 575, A98 (2015)
DOI: 10.1051/0004-6361/201425142
c ESO 2015
Impacts of pure shocks in the BHR71 bipolar outflow
A. Gusdorf1,2 , D. Riquelme3 , S. Anderl4,5 , J. Eislöffel6 , C. Codella7 , A. I. Gómez-Ruiz3 , U. U. Graf8 ,
L. E. Kristensen9 , S. Leurini3 , B. Parise10 , M. A. Requena-Torres3 , O. Ricken3 , and R. Güsten3
1
2
3
4
5
6
7
8
9
10
LERMA, Observatoire de Paris, École Normale Supérieure, PSL Research University, CNRS, UMR 8112, 75014 Paris, France
e-mail: [email protected]
Sorbonne Universités, UPMC Univ. Paris 6, UMR 8112, LERMA, 75005 Paris, France
Max Planck Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
Univ. Grenoble Alpes, IPAG, 38000 Grenoble, France
CNRS, IPAG, 38000 Grenoble, France
Thüringer Landessternwarte, Sternwarte 5, 07778 Tautenburg, Germany
INAF, Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
KOSMA, I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
School of Physics & Astronomy, Cardiff University, Queens Buildings, The parade, Cardiff CF24 3AA, UK
Received 10 October 2014 / Accepted 23 December 2014
ABSTRACT
Context. During the formation of a star, material is ejected along powerful jets that impact the ambient material. This outflow regulates
star formation by e.g. inducing turbulence and heating the surrounding gas. Understanding the associated shocks is therefore essential
to the study of star formation.
Aims. We present comparisons of shock models with CO, H2 , and SiO observations in a “pure” shock position in the BHR71 bipolar
outflow. These comparisons provide an insight into the shock and pre-shock characteristics, and allow us to understand the energetic
and chemical feedback of star formation on Galactic scales.
Methods. New CO (Jup = 16, 11, 7, 6, 4, 3) observations from the shocked regions with the SOFIA and APEX telescopes are presented and combined with earlier H2 and SiO data (from the Spitzer and APEX telescopes). The integrated intensities are compared
to a grid of models that were obtained from a magneto-hydrodynamical shock code, which calculates the dynamical and chemical
structure of these regions combined with a radiative transfer module based on the “large velocity gradient” approximation.
Results. The CO emission leads us to update the conclusions of our previous shock analysis: pre-shock densities of 104 cm−3 and
shock velocities around 20−25 km s−1 are still constrained, but older ages are inferred (∼4000 years).
Conclusions. We evaluate the contribution of shocks to the excitation of CO around forming stars. The SiO observations are compatible with a scenario where less than 4% of the pre-shock SiO belongs to the grain mantles. We infer outflow parameters: a mass
of 1.8 × 10−2 M was measured in our beam, in which a momentum of 0.4 M km s−1 is dissipated, corresponding to an energy of
4.2 × 1043 erg. We analyse the energetics of the outflow species by species. Comparing our results with previous studies highlights
their dependence on the method: H2 observations only are not sufficient to evaluate the mass of outflows.
Key words. stars: formation – ISM: jets and outflows – ISM: individual objects: BHR71 – submillimeter: ISM – infrared: ISM –
shock waves
1. Introduction
Molecular shocks are ubiquitous in the interstellar medium
(ISM) of our Galaxy. They can be associated with the formation
of “ridges” at the convergence region of molecular clouds (e.g.
W43, Nguyen-Luong et al. 2013), to the jets and outflow systems related to the formation of low- to high-mass stars (from
low, e.g. Gomez-Ruiz et al. 2013; to high, Leurini et al. 2013;
through intermediate, Gómez-Ruiz et al. 2012; for examples,
also see Arce et al. 2007; Frank et al. 2014; or Tan et al. 2014
for reviews), or to supernova remnants (SNRs) interacting with
molecular clouds (e.g. W44, Anderl et al. 2014). However, an
analysis of these shocks is challenging because several physical
processes are contributing to the observed emission. The W43
ridges are illuminated by an energetic UV radiation field coming from neighbouring H  regions of star clusters (Bally et al.
2010). Studying young stellar objects (YSO) by pointing on the
central protostar leads in practice to a study of the combination
of ejection shocks (that can be multiple, e.g. Kristensen et al.
2013), infall processes, and UV illumination (the latter being all
the more important when the mass of the forming object is large,
e.g. Visser et al. 2012; San José-García et al. 2013). Outflows
from massive star-forming regions are also illuminated by strong
radiation in the X-ray regime (e.g. W28 A2, Rowell et al. 2010).
Similarly, old SNRs are also often subject to X-rays, and also
to γ-ray emission, which are the signature of the acceleration of
particles that locally took place shortly after the supernova explosion (e.g. Hewitt et al. 2009).
“Pure” molecular shock regions can be defined as regions
where the physics and chemistry are dominantly driven by
shocks. Studying these regions, therefore, is of crucial importance to investigating the feedback they exert on their
environment, whether this feedback is energetic or chemical. From the energetic point of view, these studies allow
Article published by EDP Sciences
A98, page 1 of 14
A&A 575, A98 (2015)
us to assess the contribution of shocks to the excitation of
e.g. CO on galactic scales, as observed by Herschel (in
NGC 1068: Hailey-Dunsheath et al. 2012; M 82: Kamenetzky
et al. 2012; NGC 6240 and Mrk 231: Meijerink et al. 2013, or
NGC 253: Rosenberg et al. 2014a and Arp299: Rosenberg et al.
2014b) from the inside of the Galaxy. From the chemical point
of view, these studies allow us to investigate the formation paths
of water (e.g. Leurini et al. 2014b) or more complex molecules
(e.g. complex organic ones, Belloche et al. 2013), and to understand their presence in planetary systems, for instance. These
“pure” shock regions are not numerous. The most remarkable
and well-studied pure shock region is the B1 knot of the L1157
bipolar outflow. Sufficiently distant from the central protostar,
this region remains uncontaminated by infall or irradiation processes, and has been the subject of a number of studies dedicated
to studying its energetics or chemical composition (Gusdorf
et al. 2008b; Codella et al. 2010; Flower & Pineau des Forêts
2012; Benedettini et al. 2013; Busquet et al. 2014; Podio et al.
2014). In particular, L1157 was mapped by the German Receiver
for Astronomy at Terahertz Frequencies (GREAT) onboard the
Stratospheric Observatory for Infrared Astronomy (SOFIA) in
CO (11−10), but the low signal-to-noise ratio prevented Eislöffel
et al. (2012) from a thorough study of its energetics or chemistry.
This article focus on the analysis of a similar shock position in
the BHR71 outflow, the “southern twin” of L1157. Because of its
southern location, BHR71 is an ideal target to be observed with
the Atacama Large Millimeter/submillimeter Array (ALMA),
which will never be done for L1157 because of its high northern
location. This article is organised as follows: Sect. 2 presents our
observations. Section 3 presents the new CO data we made use of
in our analysis, while the existing H2 and SiO data is described
in Sect. 4. The results of shock modelling and their further use
is exposed in Sect. 5, and Sect. 6 summarises our findings.
2. The BHR71 bipolar outflow
2.1. Previous work
The double bipolar outflow BHR71 (Bourke et al. 1997; Myers
& Mardones 1998; Parise et al. 2006) lies close to the plane of
the sky. It is powered by two sources IRS1 and IRS2, separated
by ∼3400 AU (Bourke et al. 2001), of luminosities 13.5 and
0.5 L (Chen et al. 2008), relatively nearby (∼200 pc, Bourke
et al. 1995a). The present work builds on previous studies of
the BHR71 bipolar outflow in H2 (Neufeld et al. 2009; and
Giannini et al. 2011, hereafter N09 and Gia11), and H2 plus SiO
(Gusdorf et al. 2011, hereafter G11). In their work, N09 mostly
described the Spitzer observations of the outflow. The InfraRed
Spectrograph (IRS) was used to map the inner part of the outflow in the pure rotational transitions of H2 as well as in Fe 
and S  transitions. A region corresponding to approximately
half the length of the outflow was covered by these observations
around the driving protostars. The results were compared to previous observations of the entire region by the InfraRed Array
Camera (IRAC) onboard the same telescope, showing that 30%
and 100% of the luminosity of bands 3 and 4 could be accounted
for by H2 lines emission, similar to L1157. The pure rotational
H2 luminosity of the flow was estimated to be 4.4 × 10−2 L ,
less than 1/3 of that of L1157, but measured only from the fraction of the BHR71 outflow that was mapped. The H2 mass above
100 K was constrained to be around 2.5 × 10−3 M , 20 times less
than in L1157. However the H2 densities for both outflows were
constrained to ∼103.8 cm−3 . In Gia11, these IRS observations
of H2 were detailed line by line. An average column density of
A98, page 2 of 14
H2 of around 1020 cm−2 was extracted from the map. Two temperature components were apparent, at ∼300 and ∼1500 K. A
non-local thermodynamical equilibrium analysis was performed
and yielded a high H2 average density of a few 106 cm−3 . The
total H2 luminosity of the flow was estimated to be twice as high
as the pure rotational lines, also based on previous observations
of the rovibrational lines emission presented in Giannini et al.
(2004). In a few positions where this was possible, the observations of pure rotational H2 lines were combined with those of
rovibrational lines to generate a more complete excitation diagram that was also compared to shock models, confirming the
high value of H2 density.
In G11, the authors presented a shock-model analysis of
various positions in the northern lobe of the BHR71 outflow.
They used Spitzer observations of H2 , and APEX observations
of SiO to constrain shock models. Their analysis was hampered by the rather high beam-size of their SiO observations.
However, they identified two positions where a shock analysis
was favourable: the “SiO peak” and “SiO knot”. These positions
lie at the apex of the inner bipolar structure of the outflow, relatively un-contaminated by envelope infall. They are far away
from the protostars, and separated from them by the inner outflow cavity, hence as shielded as possible from their potential
UV radiation. For all these reasons they are reminiscent of the
L1157-B1 position, and are considered in the present study as
“pure shock” positions.
Figure 1 shows the entire BHR71 outflow as seen through
these various datasets and highlights the particular positions as
first introduced by Giannini et al. (2004): the so-called knots 5,
6, and 8 are thus indicated, as well as the two Herbig-Haro objects HH320A/B and HH321A/B. The driving protostars IRS 1
and IRS 2 are also marked. The two extra positions used in
G11, “SiO peak” (of emission) and neighbouring “SiO knot” are
also shown at the apex of the upper inner lobe of the outflow.
In this figure, the previous and most recent observations can
be compared (APEX, see Sect. 2.2): intensity in the CO (6−5)
(colours) and (3−2) (white contours) transitions integrated between −50 and 50 km s−1 in the left panel, and Spitzer IRAC
(8 µm, colours) and IRS (H2 0−0 S(5), white contours) maps
overlaid with SiO (5−4) map (red contours in the upper lobe) in
the right panel. The white contours then show how much of the
outflow was observed by the IRS onboard Spitzer.
2.2. APEX observations of CO
APEX1 observations of the entire BHR71 outflow were conducted in 2013 and 2014. The analysis of the whole maps
is out of the scope of the present work, and will be the
subject of a forthcoming publication. In the present study,
we used information inferred from the full maps in the
13
CO (3−2), 12 CO (3−2), (4–3), (6–5), and (7–6) transitions, which we briefly describe. The APEX observations towards BHR71 were conducted on several days: June 3 and 4,
2013, and June 28, 2014. We used the heterodyne receivers
FLASH345, FLASH460 (First Light APEX Submillimeter
Heterodyne receiver, Heyminck et al. 2006), and CHAMP+
(Carbon Heterodyne Array of the MPIfR, Kasemann et al. 2006;
Güsten et al. 2008), in combination with the MPIfR fast Fourier
transform spectrometer backend (XFFTS, Klein et al. 2012).
1
This publication is partly based on data acquired with the Atacama
Pathfinder EXperiment (APEX). APEX is a collaboration between
the Max-Planck-Institut für Radioastronomie (MPIfR), the European
Southern Observatory, and the Onsala Space Observatory.
A. Gusdorf et al.: Impacts of pure shocks in the BHR71 bipolar outflow
Fig. 1. The BHR71 bipolar outflow in its entirety, in left: CO (6−5) (colours, associated with the wedge, in main beam temperature units, K km s−1 )
and (3–2) (white contours, from 30% to 100% of the signal in steps of 10%), both observed by the APEX telescope and integrated between −50
and +50 km s−1 , with the resolutions indicated by green circles in the lower right corner; right: the 8 µm emission detected by the Spitzer/IRAC
receiver (colours, N09), with the H2 0–0 S(5) emission as observed by the Spitzer/IRS receiver in the inner parts of the outflow (white contours,
from G11), and the SiO (5–4) emission (red and black contours, G11) in the upper lobe (the green circles in the lower right corner show the
respective resolutions of CO (3–2), (11–10) and SiO (5–4)). On both maps, the grey inset is the field shown in Fig. 2, the SiO peak and knot
positions are indicated in blue, the knot 5 and HH320 region are in pink, and the IRS 1&2, knots 6, 8, and HH321 region are indicated by grey
dots.
The central position of all observations was α[J2000] =
12h 01m 36.s 3, δ[J2000] = −65◦ 080 53.00 0. We checked the focus
at the beginning of each observing session, after sunrise and
sunset on Saturn. We checked line and continuum pointing locally on IRC+10216, 07454-7112, or η Car. The pointing accuracy was better than ∼500 rms, regardless of which receiver
we used. Table 1 contains the main characteristics of the observed lines and corresponding observing set-ups. The observations were performed in position-switching/on-the-fly mode using the APECS software (Muders et al. 2006). We reduced the
data with the CLASS software2 . This reduction included baseline subtraction, spatial, and spectral regridding. For all observations, the maximum number of channels available in the backend
2
See http://www.iram.fr/IRAMFR/GILDAS
was used (8192). We obtained maps for all considered transitions, covering the field of the whole outflow shown in Fig. 1.
Figure 2 shows the velocity-integrated CO (6−5) broad-line
emission (colours, resolution 900 ) in the upper inner part of
BHR71, overlaid with the CO (3−2) (white contours, resolution 1800 ). The higher angular resolution of the CO (6−5) line
emission shows that it traces the walls of the cavity of the outflow associated with IRS1 (see Fig. 1), with a local maximum
of emission also corresponding to the outflow driven by IRS2
(the HH320 AB position in the map). The outflow is seen close
to the plane of the sky. The two positions of interest identified
by G11 are marked by a blue hexagon and circle corresponding to the beam of our CO (11−10) observations with GREAT
(which will be the focus of our analysis, see Sects. 2.3 and 3).
The position of the SiO peak, detected through 2800 -resolution
observations of the SiO (5–4) is localised between two emission
A98, page 3 of 14
A&A 575, A98 (2015)
was updated regularly against temperature drifts of the telescope
structure. The pointing was established with the optical guide
cameras to an accuracy of ∼500 . The line and observation parameters are listed in Table 1. The integration time was 13 min
ON source for each line, for respective final rms of ∼0.70 and
0.75 K. The data were calibrated with the KOSMA/GREAT calibrator (Guan et al. 2012), removing residual telluric lines, and
subsequently processed with the CLASS software.
3. Results: the CO data
3.1. Spectra
Fig. 2. Overlay of the velocity-integrated CO (6−5) (colour background) with the CO (3−2) (white contours) emission observed with
the APEX telescope in the inner upper lobe of the BHR71 outflow. For
both lines, the intensity was integrated between −50 and 50 km s−1 . The
wedge unit is K km s−1 in main beam temperature. The CO (3−2) contours are from 50 to 100% of the maximum, in steps of 10%. The halfmaximum contours of the CO (3−2) and (6−5) maps are indicated in red
and black, respectively. The dark blue circles indicate the positions and
beam size of the SOFIA/GREAT observations for the CO (11−10) transition. The APEX and SOFIA beam sizes of our CO (6−5), (16−15) and
(11−10) observations are also provided (upper left corner light green
circles, see also Table 1). The pink hexagons mark the position of the
so-called knot 5 and HH320AB object.
peaks in CO (6−5). The most prominent of these CO peaks is the
southern one, which corresponds to the so-called SiO knot; it coincides with an H2 emission peak (also see Fig. 5, which overlays
the same CO (6−5) field with the H2 0–0 S(5) data of N09 and
Gia11). The half-maximum emission contours are also given in
this map in thick black and red contours for the (6–5) and (3–2)
lines, respectively, at their nominal resolutions.
2.3. SOFIA-GREAT observations of CO
The observations towards BHR71 were conducted with the
GREAT3 spectrometer (Heyminck et al. 2012) during SOFIA’s
Cycle 1 “southern deployment” flight on July 23rd, 2013. We observed two positions, towards the northern apex of the inner outflow structure: the SiO knot and peak, as defined in G11 (Figs. 1
and 2). We tuned the receiver to the CO (11−10) and (16−15)
lines frequency 1267.014 GHz LSB and 1841.346 GHz USB.
We connected the receiver to a digital FFT spectrometer (Klein
et al. 2012) providing a bandwidth of 1.5 GHz with respective
spectral resolutions of 0.018 and 0.012 km s−1 . The observations
were performed in double beam-switching mode, with an amplitude of 8000 (or a throw of 16000 ) at the position angle of 135◦
(NE−SW) and a phase time of 0.5 s. The nominal focus position
3
GREAT is a development by the MPI für Radioastronomie
and the KOSMA/Universität zu Köln (Kölner Observatorium für
SubMillimeter Astronomie), in cooperation with the Max-PlanckInstitut für Sonnensystemforschung and the Deutsches Zentrum für
Luft- und Raumfahrt Institut für Planetenforschung.
A98, page 4 of 14
Figure 3 presents all 12 CO spectra obtained in the two targeted
positions in the course of our APEX (Jup = 3, 4, 6, 7) and SOFIA
(Jup = 11, 16) observations, after convolution to the same angular resolution, that of the CO (11−10) transition (see Table 1 for
observing parameters related to each of these lines). The only
exception to this convolution is the CO (16−15) line, for which
only a single-point observations is available. As the line was observed at an angular resolution of roughly 16.00 3, we could in
principle use the integrated intensity extracted from this line as
an upper limit to that convolved to the 2400 resolution. In fact, we
corrected this value for the beam dilution effect in our analysis,
see discussion in Sect. 5.1 on how we made use of this line. On
the figure, we split the spectra in two groups for visibility purposes (Jup = 3, 4, 6 in the upper panels, and Jup = 6, 7, 11, 16 in
the lower panels).
In both positions, all lines exhibit a profile typical of the presence of a pure shock, with wings extending towards red-shifted
velocities, up to 30–40 km s−1 ; up to Jup = 6, self-absorption is
also detected at the velocity of the cloud (around −4.5 km s−1 ).
The CO (3–2) and (4–3) profiles coincide very well, suggesting that these lines are optically thick. This is confirmed for the
CO (3−2) line, for which we observed the 13 CO isotopologue
on both positions (see Sect. 3.2, Table 1 and Fig. 4 for the spectra obtained on both positions). In both positions, the CO (6–5)
profiles start to differentiate from the lower-lying profiles. This
departure is confirmed in the higher-lying transitions, and reveals that the lines from Jup = 6 are probably excited in different layers of the shocked region than their lower-lying counterparts. Globally, it can be seen that the excitation conditions vary
with the position: the Jup = 11 and 16 profiles differ, and the
CO (16–15) is not even detected in the SiO peak position. For
both positions, the excitation conditions also vary with the velocity: close to the systemic velocity, the lower-J line emission
greatly dominates, an effect previously reported in L1157 B1
by Lefloch et al. (2012). Because of the associated weak or
absent detection in the SOFIA lines, and because the two selected positions are not independent, we decided to exclude the
SiO peak from the shock analysis presented in Sect. 5 (also see
an additional argument in Sect. 4.1).
3.2. CO (3−2) line opacities
Figure 4 shows our spectra of the 13 CO and 12 CO lines in the
SiO peak and knot positions at the same spectral and spatial
resolutions. These spectra allow us to plot the ratio of the line
temperature. For each velocity channel, the line temperature ratio 12 CO/13 CO is hence also shown (right ordinates) with the
following colour-code: red dots show the ratios for the velocity channels where the 13 CO is detected at more than 3σ, and
orange dots where the 13 CO detection was obtained with a confidence level between 2 and 3σ. The grey dots are for all the
A. Gusdorf et al.: Impacts of pure shocks in the BHR71 bipolar outflow
Table 1. Observed lines and corresponding telescope parameters for the APEX and SOFIA observations of BHR71.
Species
Line
Telescope
ν (GHz)
FWHM (00 )
Sampling (00 )
Receiver
Observing days
Feff
Beff
T sys (K)
∆3 (km s−1 )
Reference offset (00 )
12
CO
(3–2)
APEX
345.796
18.1
6
FLASH345
2013-06-03
2013-06-04
0.95
0.69
169−193
0.331
(–120, 260)
12
CO
(4–3)
APEX
461.041
13.5
6
FLASH460
2013-06-03
2013-06-04
0.95
0.60
408−610
0.496
(–120, 260)
12
CO
(6–5)
APEX
691.473
9.0
4
CHAMP+
2013-06-04
2014-06-28
0.95
0.42
3650−10 000
0.635
(–120, 260)
12
CO
(7–6)
APEX
806.652
7.7
4
CHAMP+
2013-06-04
2014-06-28
0.95
0.35
12 000−34 000
0.544
(–120, 260)
12
CO
(11–10)
SOFIA
1267.014
24
pointed
GREAT
2013-07-23
2013-07-23
0.97
0.67
3274–3307
0.018
beamswitch
12
CO
(16–15)
SOFIA
1841.346
16.3
pointed
GREAT
2013-07-23
2013-07-23
0.97
0.67
3160–3207
0.012
beamswitch
13
CO
(3–2)
APEX
330.588
18.9
6
FLASH345
2013-06-03
2013-06-04
0.95
0.69
182-224
0.346
(–120, 260)
Fig. 3. CO transitions in the SiO peak (left panels) and knot (right panels) positions indicated in Fig. 2: APEX (3–2), black line; (4–3), red line;
(6–5), green line and histograms (upper panels); (7–6), dark blue lines; SOFIA (11–10), pink lines and (16–15), grey lines (lower panels, overlaid
on the green histograms of the 6–5 transition). The last two were multiplied by five for comparison purposes. Respective spectral resolutions are
0.33, 0.50, 0.64, 0.54, 0.90 and 0.62 km s−1 . The vertical dotted line marks the cloud velocity, −4.5 km s−1 (Bourke et al. 1995b).
other channels. Horizontal dashed lines show the reference values of 20 and 40 for these ratios. The orange and red dots all lie
below 30. Assuming an identical excitation temperature for the
12
CO and 13 CO lines, and a typical interstellar abundance ratio
of 50–60 (e.g. Langer & Penzias 1993), these line ratios yield
minimum opacity values of 3 for the 12 CO lines. We therefore
conclude that the emission is optically thick at least in the lowvelocity regime of the spectral wings. The large optical thickness is consistent with the constancy of the line integrated intensities (in the non-absorbed components) from CO (3–2) and
(4–3), when convolved to the same resolution(s). As our observations yield a minimum optical thickness of 3 up in the wings,
we decided to exclude both the CO (3–2) and (4–3) lines from
our analysis.
3.3. Dynamical age of the outflow
An important parameter for the modelling of young outflows is
their age. As we have mapped the entire outflow, we are able
to give an upper limit of its age, simply obtained by dividing
the distance between the furthest point from the driving protostar and the protostar by the associated linewidth. The positions
we consider belong to the northern lobe of the outflow powered by the IRS 1 protostar. As the furthest point with significant
CO emission belonging to this outflow, we adopted the furthest
point on the 10% CO (3–2) emission contour or the furthest point
with 3σ emission away from the driving protostar, and found that
these two approaches were equivalent. Indeed, the offset from
both points relative to the IRS1 position is about (−7200 , 28000 ),
A98, page 5 of 14
A&A 575, A98 (2015)
Fig. 4. Comparison of the 12 CO (black line) and 13 CO (pink line) (3−2)
emission profiles obtained in the SiO peak (upper panel) and knot
(lower panel) positions. The ratio of main-beam temperatures is also
shown in the form of red dots (for the channels where the 13 CO detection is over 3σ), orange dots (for the channels where the 13 CO detection is between 2 and 3σ), and grey dots (for the remaining channels).
Associated with this distribution are the grey horizontal dotted lines,
which show values of 20 and 40 for this ratio.
corresponding to a distance of ∼28900 . At a distance of 200 pc,
this translates in ∼5.8 × 104 AU. In this position, the full-width
at zero intensity of both the CO (3–2) and (6–5) (unshown) lines
is about 15−20 km s−1 , which converts to a dynamical age of
(1.4−1.8) × 104 years. Of course, this is an upper limit of the real
age of the outflow, as it is likely that the apex of the outflow was
associated with greater velocities in the past. A similar measurement for the SiO knot position (i.e. dividing its distance to the
protostar by the full-width at zero intensity) yields a dynamical
age of ∼1800 years.
4. Results: existing data
4.1. H2 observations
We used the Spitzer/IRS observations of the H2 pure rotational transitions, 0–0 S(0) up to S(7), reported and analysed
in N09, Gia11, and G11. The reduced data kindly communicated to us by David Neufeld contain rotational transition maps,
with 3.00 6 angular resolution, centred on α[J2000] = 12h 01m 36.s 31,
δ[J2000] = −65◦ 080 53.00 02. Figure 5 shows an overlay of our
APEX CO (6−5) map with the H2 0−0 S(5) region observed by
Spitzer, both at their nominal resolution. The figure shows coinciding maxima between the two datasets in the selected position,
A98, page 6 of 14
Fig. 5. Overlay of the map of CO (6–5) emission observed by the
APEX telescope (colour background) with the H2 0–0 S(5) emission
(white contours), observed with the Spitzer telescope. The wedge unit
is K km s−1 (main beam temperature) and refers to the CO observations. The H2 0–0 S(5) contours are from 10% to 100%, in steps of
10%. The light blue contour defines the half-maximum contour of this
transition. Like in Fig. 2, the blue circles and dots indicate the positions
and beam sizes of the SOFIA/GREAT observations. The beam and pixel
sizes of the CO (6–5) (11–10), (16–15), and H2 0–0 S(5) observations
are the green circles in the upper left corner. The black contour delineates the half-maximum contour of the H2 0–0 S(2) transition. The field
is the same as in Fig. 2, and the knot 5 and HH320 region are in pink.
The field covered by Spitzer/IRS to observe the H2 emission is indicated
in dashed grey line. It excludes the SiO peak position.
and a slightly different emission distribution. This might be the
effect of the better spatial resolution of the H2 data, which reveal
more peaks than in CO. This overlay also shows the similar morphology of the emission of the S(2) (half-maximum contour in
black) and S(5) (half-maximum contour in light blue) transitions
on the region we chose to analyse. Unfortunately, the figure also
shows the coverage of the H2 emission observations, and that
the SiO peak position was not covered by the H2 observations,
which is another reason to exclude it from our shock analysis
presented in Sect. 5.
As part of our modelling (see Sect. 5), we used the excitation diagram derived for the selected emission region around the
SiO knot position. The H2 excitation diagram displays ln(N3 j /g j )
as a function of E3 j /kB , where N3 j (cm−2 ) is the column density
of the rovibrational level (v, J), E3 j /kB is its excitation energy
(in K), and g j = (2 j + 1)(2I + 1) its statistical weight (with I = 1
and I = 0 in the respective cases of ortho- and para-H2 ). If the
gas is thermalised at a single temperature, all points in the diagram thus fall on a straight line.
The initial resolution of the maps is 3.00 6, but we wanted
to operate at the same resolution as for CO. Unfortunately,
the SiO knot position lies at the edge of the field covered
in the H2 map, as can be seen in Fig. 5. We consequently
used three different methods for extracting the H2 line fluxes.
First, we extracted the flux from the initial map at its nominal
A. Gusdorf et al.: Impacts of pure shocks in the BHR71 bipolar outflow
Fig. 7. The integrated intensity diagram generated from the G11 observations of three SiO lines (Jup = 5, 6, 8), corrected for beam size and
for filling factor effect: filling factor of 1, 0.5, and 0.2 in black, dark
grey, and light grey squares. The result of our best-fit model for the H2
and CO lines (nH = 104 cm−3 , b = 1.5, 3s = 22 km s−1 , and an age of
3800 years) is shown with 1, 2, or 4% of the pre-shock Si placed in the
grain mantles in red empty, dotted, or filled circles.
Fig. 6. Our model-observations comparisons. Upper panel: CO flux diagram over a beam of 2400 ; the observations in the SiO knot position are
the black squares, and the model results are the coloured circles (see
text). The CO (16−15) observational point is corrected for beam-size
effect. The uncorrected point is the upper limit (see text), indicated by
the grey arrow in this panel. Lower panel: H2 excitation diagram for
the SiO knot position, extracted following the different procedures described in the text (empty symbols), global average in black squares,
and model results in coloured circles (see text, with the same code as in
the upper panel).
3.00 6 resolution; second, we used the first method, but applied to
a map that was convolved to the 2400 resolution; and third, we
associated a filling factor of 0.2 to the fluxes obtained through
the second method.
Because of the location of the SiO knot and the rather large
beam of our analysis, the first two methods provide lower limits
to the H2 line fluxes. On the contrary, with method (3) we voluntarily extracted upper limits to these fluxes. We then used an average value between the values inferred from the three methods,
and computed rather large errorbars based on the combination
of all three methods. The values were corrected from interstellar
extinction following the treatment already applied in G11. The
result can be seen in the excitation diagram shown in the lower
panel of Fig. 6, where points corresponding to methods (1), (2),
(3) are shown in black empty diamonds, upward and downward
triangle, and the resulting average points are the black squares,
respectively. The overall average H2 values we used to build our
figure are given in Table B.1.
4.2. SiO
As part of our study, we also decided to re-analyse the SiO observations already presented in G11. The emission from three
lines was mapped in the northern lobe of the outflow: SiO (5–4),
(6–5), and (8–7), and the corresponding spectra were extracted
in the SiO peak and knot positions. As maps were obtained,
all spectra were convolved to the resolution of the SiO (5–4)
line, about 2800 . In the present study, we hence corrected the
SiO dataset for the slight difference in resolution between this
value and that of the CO (11−10) observations, 2400 , by simply multiplying the G11 integrated intensities by (28/24)2 . Also,
given the rather large beam Rsize, we generated an integrated intensity diagram (displaying T MB d3 against Jup for each transition) for three different filling factor values, 1, 0.5, and 0.2. The
result can be seen in the form of respective black, dark grey and
light grey squares in Fig. 7. The SiO values we used to build our
figure are listed in Table B.1.
5. Discussion
In the following we seek to constrain the physical conditions in
the shocked regions. Given the large number of observational
constraints, covering several species and several associated
transitions, the method-of-choice for obtaining these physical
conditions is a comparison to sophisticated shock models. The
results of the shock modelling forms the foundation of the discussion: what characterises the chemistry of the outflow, in particular with respect to SiO, and what characterises the energetics.
5.1. Constraining shock models
Shock models are used to constrain the physical conditions in
the outflow shocks through comparison with H2 and CO, following the methods already used in Gusdorf et al. (2008a, 2012)
and Anderl et al. (2014). We generated CO flux diagrams and
H2 excitation diagrams from the observations of the SiO knot
position, and compared them with results from a grid of models
computed with the “Paris-Durham” (e.g. Flower & Pineau des
Forêts 2003) 1D shock model. The H2 diagram has already been
introduced in Sect. 4.1 and is shown in Fig. 6. Concerning the
observable associated with CO, we chose to display the data in
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A&A 575, A98 (2015)
Table 2. Shock model parameters.
Shock type
a
C-type
C-typeb
J-type
CJ-type
Number of models
108
9
21
1035
3 [km s−1 ]
20–55
20–40
10–50
10–50
∆3 [km s−1 ]
2–5
10
2 –15
2 –5
b
0.45–2
1–3
0.1
0.3 –2
∆b
0.15–0.25
0.5
–
0.15–0.25
nH [cm−3 ]
3
4
5
6
10 , 10 , 10 , 10
105
104 , 105
103 , 104 , 105
Age [yr]
∆Age [yr]
–
–
–
4–15 000
–
–
–
1–1285
Notes. Grid intervals ∆x are listed as minimum and maximum increments found in the grid.
References. (a) Gusdorf et al. (2008b, 2011, 2012); (b) Anderl et al. (2014).
the form of a flux diagram (line flux vs. Jup ), because of the increasing number of extragalactic studies that use this form. In the
present case, we benefit from the fact that our observations were
pointed towards a pure shock position. To generate a CO flux diagram, we consequently integrated the signal obtained over the
whole line profile at the angular resolution of 2400 , associated
with a filling factor of 1 (see Appendix A for an explanation on
filling factors for various CO transitions). Because of their opacity, and their likeliness to be contaminated by ambient gas, we
excluded the (3–2) lines from our analysis (see Sect. 3.2). Given
the opacity values inferred from this line, we also excluded the
(4−3) line from our fitting procedure. Overall, the two observational tools we used can be seen in Fig. 6: a flux diagram for
CO (upper panel) and an excitation diagram for H2 (lower panel)
on which the observational points for the SiO knot are always
shown in black squares. The CO values we used to build our figures are listed in Table B.1. The observational flux diagram for
the SiO peak position can be found in Appendix C (Fig. C.1).
The grid of shock models is that of G11, also used in
other publications already mentioned (Gusdorf et al. 2012;
Anderl et al. 2014). To summarise, we used a grid covering the
following input parameters: pre-shock density nH from 103 to
106 cm−3 , magnetic field parameter from b = 0.3 to 2 (defined
by B(µ G) = b × [nH (cm−3 )]1/2 , where B is the intensity of
the magnetic field transverse to the shock direction of propagation), and shock velocity 3s from 15 to 35 km s−1 . Our grid
contains both stationary C- and J-type models, and also nonstationary, so-called CJ-type models (Lesaffre et al. 2004a,b). A
more complete description of the parameters space coverage (nH ,
b, and 3s ) can be found in Table 2. The parameter coverage is
not complete: not all velocities are present in our grid for all
values of the magnetic-field parameter, b. In fact, the velocity
of C-type shocks must remain below a critical value that depends mainly on the pre-shock density and magnetic-field parameter (Le Bourlot et al. 2002; Flower & Pineau des Forêts
2003), which explains the decrease of the maximum shock velocity with the pre-shock density in our grid. Additionally, the
time necessary to reach a steadystate depends on the pre-shock
density, shock velocity, and magnetic field values. Following the
method presented in G08b (Sect. 4.1), the set of C-type shock
models enabled us to restrict the range of the search in the parameter space for the CJ-type shock models. We then computed
a grid of non-stationary shock models around a first estimate of
the shock age, making sure that the range of ages was sufficient
to include any model likely to fit the H2 observational data. The
final considered ages range from a few tens to around fifteen
thousand years. Our grid also includes a few stationary models
including the effects of grain-grain interactions, as presented in
Guillet et al. (2011); Anderl et al. (2013), and already used in
Anderl et al. (2014); Leurini et al. (2014a).
We compared models and observations based on the three
higher-J CO lines, from CO (6−5) to (11–10), and on all
A98, page 8 of 14
the H2 pure rotational lines. The CO (3−2) and (4–3) lines
were excluded from our procedure, as stated above. Our initial
CO (16−15) observation is in principle an upper limit, since it
was associated with a slightly smaller beam than the other lines.
Nevertheless we assumed that the CO (16−15) emission is as extended as the CO (6−5) emission, and we estimated the resulting
flux over a 2400 beam by multiplying the value observed over a
16.00 3 beam by a factor (16.3/24)2 . We then treated this corrected
value as any other CO observation. Similar to what we did in
previous studies, we used a χ2 routine to compare these observations to the models. The best results can be seen in Fig. 6.
On each of these panels, we show the points of our best fit in
red circles, plus three other satisfying models in smaller, purple,
green, and blue circles. Our best-fit model (in red points) is nonstationary, with the following characteristics: nH = 104 cm−3 ,
b = 1.5, 3s = 22 km s−1 , and an age of 3800 years. The χ2 value
of our 10 and 20 best-fit models are within a factor 1.4 and 2.1 to
the best (lowest) value. Broadly speaking, we found the H2 lines
in particular very difficult to fit: the exact curvature of the excitation diagram is only approached by our best model. We tried
in vain to improve the quality of the fit by changing the ortho-topara ratio of H2 in our calculations (switching its value from 3
to 1 and 2). Eventually, we can not exclude that a better fit could
be found via a better gridding of our parameters space. However,
it turned out that this best-fit model also fitted the CO (3−2)
and (4−3) lines quite in a very satisfying way. These values can
be compared to what was constrained by G11: nH = 104 cm−3 ,
b = 1−2, 3s = 25−30 km s−1 , and an age of 300−800 years. A
few comments can be made on these shock parameters.
Shock type. The shock type we constrained is the same as
found by G11. Indeed, it is a very general result that in lowmass star forming environments, H2 excitation diagrams can be
fitted by these kinds of models only (e.g. Giannini et al. 2006;
Gusdorf et al. 2008b; Flower & Pineau des Forêts 2013). All
of our ten best fits are CJ-type models. Even more, the J- and
C-type models are all at the end of our list of best-fit models: the
C-type models generally do not fit the pure rotational H2 excitation diagram, while the J-type models do not generate enough
CO emission to match the observations.
Pre-shock density. First, the pre-shock density value is the
same as that found in previous analyses of BHR71 (Giannini
et al. 2004, for the analysis of the HH320 region, G11 for the
SiO knot position), and in the resembling pure shock position
L1157-B1 (e.g. Gusdorf et al. 2008b; Flower & Pineau des
Forêts 2012). It is also the value associated with our ten best
fits to the dataset; it is relatively well constrained, as in particular the general shape of the CO diagram is very sensitive to the
density, i.e. to the pre-shock density parameter. In our ranking
of models by decreasing χ2 value, the first model with a different pre-shock density has a χ2 value that is more than five
times the lowest one. Generally speaking, it is a very standard
value when modelling shocks from low-mass YSOs (also see
A. Gusdorf et al.: Impacts of pure shocks in the BHR71 bipolar outflow
HH54, Giannini et al. 2006), which was also found in shocks
associated with SNRs that are interacting with the interstellar
medium (Cesarsky et al. 1999; Gusdorf et al. 2012; Anderl et al.
2014). This corresponds to a post-shock density of ∼105 cm−3 , a
value that is one or two orders of magnitude smaller than those
found by Gia11 in the HH320 and 321 regions of the same outflow. This could be due to the fact that their study only focussed
on the brightest pixel associated with these regions, whereas we
consider a rather large beam size. On the other hand, our value is
just above the H2 density averaged over the whole inner outflow
found by N09 (103.8 cm−3 ). This can be explained by the fact we
are focussing on a bright H2 spot, and also that the post-shock
value we are providing here is that of a stationary post-shock. In
the (realistic) case where the shock has a finite spatial size, the
post-shock gas will expand and come into pressure equilibrium
with its surroundings, resulting in a globally lower value than
ours.
Magnetic field strength. The strength of the transverse magnetic field is 150 µG in the pre-shock region, that turns into
1.62 mG in the post-shock, given the compression factor and the
fact that the magnetic field is frozen in the neutral fluid. It is comparable to what was constrained in G11 (b = 1−2), and also to
what was found in the studies of HH320 in BHR71, L1157-B1,
or HH54, or in the aforementioned SNR studies. This value is
also consistent with the analysis of Zeeman measurements in
molecular clouds where magnetic and kinetic energy densities
are in approximate equipartition (Crutcher 1999). Our ten best
fits all predict a value of the b parameter between 1.0 and 2.0.
Figure 6 shows the best model with a b value outside of this
range, in purple points. This model is associated to a χ2 value
that is 1.8 times the lowest. This model is also a bit older than
our ten best-fit ones (see below). It does not fit the lower-J data
as satisfyingly as our best-fit model (red points in Fig. 6).
Shock velocity. The shock velocity values of our ten bestfit models all are between 20 and 25 km s−1 . In our ranking of
models by decreasing χ2 value, the first model with a velocity
out of this range has a χ2 value that is about three times the
lowest one. More specifically, the shock velocity that we constrain is slower than, e.g. the full-width at zero intensity of the
CO lines. The modelled shock velocity is the difference between
the pre-shock velocity and the impact velocity. Consequently, if
the pre-shock material is already accelerated, the actual shock
velocity is expected to be correspondingly lower. Preliminary
acceleration of the pre-shock is possible in the SiO knot position, as it has already been encompassed by the shock that is
now propagating at the northern tip of the outflow (a bit more
than twice as far from the driving protostars). Alternatively or
simultaneously, this velocity discrepancy could be the sign of
a limitation to our models. Indeed, we are modelling a multidimensional complex shock region, which is seen edge-on, by
means of a one-dimension model, which is considered to be
face-on. Additionally, it is likely that the rather large beam of
our observations contains not one shock structure, but a collection of them. This can be seen in the spatial sub-structure of
the H2 emission revealed in Fig. 5, where multiple peaks are
detected. This can also be seen in the CO (Figs. 3 and 4) or
SiO (G11) line profiles, where a “plateau” or a “bump” can be
seen between 20 and 40 km s−1 , very much resembling the bullets revealed in the CO (in e.g. Cep E, Gómez-Ruiz et al. 2012)
or water line profiles (in e.g. L1448-mm, Kristensen et al. 2011).
Another expression of the intrinsic multi-dimensional nature of
the observed shocks is the impossibility of fitting the different
molecules with the same filling factor values. For instance, H2 is
only associated with the hottest regions in our beam, as a 500 K
temperature is necessary to populate its levels, whereas CO is
more easily excited (for Jup = 6, Eup = 116 K). In particular,
unlike H2 , CO is probably emitting in the post-shock expansion
region mentioned in the “pre-shock density” paragraph above,
where the primary shock structure progressively mixes with the
ambient medium in all spatial dimensions. This effect cannot be
accounted for by a 1D shock model.
Shock age. Finally the shock age is rather large: it is larger
than what was previously constrained for this region of BHR71
by G11 (300–800 years), which is due to the high amounts of CO
that we detected, that are not compatible with younger CJ-type
shocks. However, again the age predicted by our ten best-fit
models consistently lies between 3500 and 5500 yr, with the
model in purple in the Fig. 6 being slightly older. In our ranking of models by decreasing χ2 value, the first model with an
age younger than 3000 years has a χ2 value that is about twice
the lowest. The age that we find is between the dynamical age of
the SiO knot and that of the global northern lobe (see Sect. 3.3),
which hints again at the fact that we are probably catching several shock episodes in our beam.
Limitations. The question of the largest source of uncertainty
in these results is difficult to assess. The observational problems (the different beam sizes and the fact that the SiO knot lies
close to the edge of the H2 observations) make for an important
limitation, but we believe we have taken it into account by using
conservative errorbars. More problematic is the complex nature
of shocks structures. Lacking high angular resolution, we could
only assume that we catch a single shock structure in our beam.
Our results seem to indicate this is not the case, given the constrained shock velocity and ages, and the different filling factors
we had to adopt for each molecule. A shock similar to those
we have in our grid of models is probably propagating in one
beam of 2400 , but it is very likely to be associated with less violent shocks corresponding to the mixing of its warm, dense,
and accelerated post-shock with the surrounding ISM. To summarise, we could only be aware and accept the fact that we are
averaging processes out by working over a beam size typical of
single-dish observations. From the modelling point of view, solutions do exist to more realistically account for the observations
of complex geometrical shock structures. The first consists of
adopting a probability density function (PDF) of shocks, which
is to consider that the observed shock characteristics follow a
statistical distribution. This distribution can unfortunately only
be guessed, and hence generates additional free parameters (see
Lesaffre et al. 2013 for applications of this method). The second consists of stitching 1D shock layers (similar to the one we
consider here) to either a curve (e.g. Kristensen et al. 2008) or
a bow-shock structure (e.g. Gustafsson et al. 2010), hence simulating “pseudo-” 2 or 3D shock propagation. This approach also
yields free parameters, such as the inclination of the magnetic
field with respect to the shock structure. None of these methods
would be ideal in the present case, given the lack of knowledge
of the small-scale shock structure within the 2400 beam (no indication on the collection of shocks is available at the moment),
and the relatively limited dataset (if more free parameters are to
be considered, more constraints are needed to lift the degeneracy in the results). Their application will become relevant when
the SiO knot is observed at higher angular resolution (in CO and
H2 ), and if possible with a maximum number of observations in
key species (such as H2 , CO, and SiO, but also H2 O, OH and O 
for instance). We note that at this point, a more tightly sampled
shock model will prove necessary.
In the following we will present results based on our best
fit only. All of our ten best-fit models were found to be
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non-stationary (CJ-type), with nH = 104 cm−3 , b = 1−2, 3s =
20−25 km s−1 , and an age of 3500−4500 years.
5.2. Employing shock models: SiO chemistry
After constraining shock models based on CO and H2 observations, it is now possible to fine-tune the SiO chemistry. SiO has
long been interpreted as a tracer of C-type shocks with a velocity greater than 20−25 km s−1 (e.g. Caselli et al. 1997; Schilke
et al. 1997; Gusdorf et al. 2008a). Indeed, in these types of
shocks, the drift velocity between the charged grains and the
most abundant, neutral molecular species is sufficient to generate the sputtering of the core of the grains, where all the siliconbearing material was considered to be locked. However Gusdorf
et al. (2008b) have demonstrated: 1) that CJ-type shock models were the only ones to fit the H2 emission in L1157-B1; and
2) that this process was not efficient enough to generate levels
of SiO emission comparable to the observations in this kind of
(CJ-type) shock models. In order to be able to self-consistently
account for both SiO and H2 emission, one solution was considered, consisting of transferring a fraction of SiO from the core to
the mantle of the grains in the pre-shock phase. The maximum
fraction of SiO thus transferred to the mantles was set to 10%.
With mantle sputtering being easier than core sputtering, these
authors were then able to fit the H2 and SiO emission by means
of a single, non-stationary shock model. The presence of siliconbearing material in the grain mantles has also been assumed in
Coutens et al. (2013) to explain the narrow emission component
detected in the SiO (2–1) line profile at the systemic velocity
around NGC 1333 IRAS 4A.
Interestingly, the transfer of Si from the core to the mantles
in the form of SiO was also considered by Anderl et al. (2013) in
the context of denser shock models. Indeed, for pre-shock densities above 105 cm−3 , the effect of grain-grain interactions cannot
be neglected in shock models, as shown by Guillet et al. (2011).
Taking these interactions into account results in the significant
production of small, charged grains that couple very well with
the neutral fluid, in effect reducing the width of the shock layer,
and increasing its maximum temperature. In the narrow shock
layers thus produced, Anderl et al. (2013) have shown that the
grain core sputtering is not efficient enough to produce levels of
SiO emission comparable to the observations, hence the recourse
to the transfer of a fraction of Si towards the mantles in the preshock phase. Leurini et al. (2014a) have validated this approach
by successfully confronting these models with observations.
Consequently, we ran our best-fit model for the H2 and
CO data (nH = 104 cm−3 , b = 1.5, 3s = 22 km s−1 , and an
age of 3800 years), including 0 to 10% of the pre-shock siliconbearing material in the form of SiO in the grain mantles. We
then generated the corresponding integrated intensity diagrams,
which we compared with the observations. The result can be
seen in Fig. 7. For a filling factor value conservatively varied between 0.2 and 1, we show that no more than 4% of SiO needs to
be placed in the grain mantles to reproduce the observations. The
presence of silicon-bearing material on the grain mantles could
be explained by the fact that the SiO knot position has already
been processed in the past by the shocks that are now located
at the apex of the northern outflow lobe, further north, as can be
seen in Fig. 1. Under this assumption, we have demonstrated that
it is possible to self-consistently fit H2 , and velocity-resolved CO
and SiO observations in the “SiO knot”, pure shock position of
BHR71.
A98, page 10 of 14
5.3. Employing shock models: energetics
In this section, we discuss the energetics associated with the
shocks along two axes: the CO excitation generated from our
best-fit model, and the derivation of the outflow parameters.
CO excitation. Indeed, the excitation of CO has been intensively used in the literature to demonstrate the presence of various processes at work in the regions of star formation. For example, van Kempen et al. (2010b) and Visser et al. (2012) obtained
CO flux diagrams (similar to our Fig. 6) with PACS, centred
on low-mass protostars at the origin of various outflows (HH46,
NGC 1333 IRAS 2A, and DK Cha). Based on these diagrams,
they evidenced the existence of three physical components corresponding to three distinct processes contributing to the excitation of CO: a passively heated envelope, UV-heated outflow
cavity walls, and small-scale shocks along the cavity walls. The
SiO knot position is distant from the central protostars IRS 1
and IRS 2 (see e.g. Fig. 1), which most likely prevents any UV
heating from the central star to be operating. Furthermore, it lies
outside the envelope, as revealed by IRAC (Tobin et al. 2010),
NH3 (1,1) (Bourke et al. 1995a, 1997) or N2 H+ (Chen et al.
2008) emission. In other words, The SiO knot position offers the
opportunity to study pure small-scale shocks along cavity walls.
However, the comparison of our shock models with those presented in van Kempen et al. (2010b) or Visser et al. (2012) is not
really meaningful. These authors indeed used a multiple-shock
layers model, with a pre-shock density of 106.5 cm−3 close to the
protostar, and 104.5 cm−3 further away along the envelope (see
Fig. 4 of Visser et al. 2012). Furthermore the 1D shocked layers they used were outputs of the Paris-Durham model we are
also using in the present study. However they did not include
the tip of the envelope that would correspond to the SiO knot
position, as they solely focus on the outflow cavity in the vicinity of the protostar. The conclusion from our analysis is that our
constrained pre-shock density of 104 cm−3 is consistent with the
range of values they used (as they considered a decreasing density further away from the protostar).
Another, more classical approach to discuss energetics and
physical conditions based on CO excitation consists of using
excitation diagrams, as reviewed in Visser (2014). Figure 8
presents the excitation diagram built from our best-fit model for
CO. As extensively described in Goldsmith & Langer (1999), in
local thermodynamical equilibrium conditions and under optically thin regime, the points in this type of diagram are expected
to fall on a straight line, whose inverse of the slope is the excitation temperature of the transitions (also see the description
of H2 excitation diagrams in Sect. 4.1). As pointed out in Visser
(2014), numerous studies can be found in the literature describing the building of this kind of excitation diagrams based on
PACS observations centred on outflow-driving protostars of lowto high-masses, and their subsequent decomposition in at least
two gas components, one warm (T ex = 320 ± 50 K), and one hot
(T ex = 820 ± 150 K). Usually the breakpoint between these two
components is around 1800 K, between the (25−24) and (26−25)
transitions. We have produced a similar diagram from our bestfit model, as can be seen in Fig. 8. For comparison purposes, we
have also applied a two-component linear fit of the lines accessible to PACS, with the warm component (T ex = 216 K) fitting
the level populations for Jup = 12 to 25, and a hot component
(T ex = 422 K) fitting the level populations for Jup = 26 to 40.
The values we constrained for the excitation temperatures within
these two components are systematically below those inferred
from PACS observations centred on outflow-driving protostars
A. Gusdorf et al.: Impacts of pure shocks in the BHR71 bipolar outflow
Table 3. Outflow parameters over the beam of our observations in H2 ,
CO, and SiO.
Fig. 8. The CO excitation diagram produced from our best-fit model
(red squares). Two temperature components can be fitted to our shock
model over the PACS range of observations: a warm component (T ex =
216 K, dark blue line) fitting the level populations for Jup = 12 to 25
(light blue squares), and a hot component (T ex = 422 K, light green line)
fitting the level populations for Jup = 26 to 40 (dark green squares).
of all possible mass (van Kempen et al. 2010a; Herczeg et al.
2012; Goicoechea et al. 2012; Manoj et al. 2013; Karska et al.
2013, 2014; Green et al. 2013a,b; Dionatos et al. 2013; Lee
et al. 2013; Lindberg et al. 2014). This shows that pure shocks
contribute to both the so-called “warm” and “hot” components.
Although the excitation temperature associated with the warm
component is close to the observed values, the figure also shows
that pure shocks fail to entirely account for any of these warm
or hot components. Indeed the excitation temperatures that we
find here are less than those derived from the observations in
those papers. This means that close to the protostar, additional
mechanisms, or different kinds of shocks should account for the
CO observations. It also shows to which extent the collection
of continuous temperature components that is generated by our
shock models can be interpreted as a two-excitation component,
through the analysis of CO excitation diagrams, for transitions
with Jup = 12 to 40.
Outflow parameters. Finally, we studied the energetics associated with this pure shock position. Our first method is to
follow the procedure presented in Anderl et al. (2014) for the
W44 SNR shock study. Indeed, we can infer the mass, momentum, and energy associated with our best-fit model, under the
assumption of a filling factor equal to one. The results are presented in the second column of Table 3. First, the total mass
contained in the beam is 1.8 × 10−2 M . This value is far greater
than that determined for the mapped area (half of the inner part
of the outflow) by N09 (2.5×10−3 M ), or Gia11 (0.6×10−2 M ).
This might be partly explained by the fact that our beam size is
rather large and that we decided to focus on the brightest H2
spot in the map, contrary to those studies that operated on values averaged over larger areas. More convincingly, this is probably due to the fact that our observations–models approach gives
us access to the population of the (v = 0, J = 0, 1) H2 levels, contrary to the observations. The contribution of these two
levels to the total column density is significant: a linear fit to
the Spitzer observations presented in Fig. 6 yields a column
density N(H2 ) ∼ 6.9 × 1019 cm−2 , whereas linearly fitting the
(v = 0, J = 0, 1) part of the modelled rotational diagram yields
N(H2 ) ∼ 1.8 × 1021 cm−2 , which is 26 times larger than that
measured based only on the observations. Moreover, our value
is consistent with the total mass of 1M for the northern lobe
Species
Model
H2
CO
SiO
N (cm−2 )
M (M )
δ3max (km s−1 )
td (yr)
˙ (M yr−1 )
M
P (M km s−1 )
Fm (M km s−1 yr−1 )
Ek (erg)
Lmech (L )
3.7e21
1.8e-2
–
–
–
0.4
–
4.2e44
–
1.9e21
1.3e-2
40
1785
7.0e-6
5.0e-1
2.8e-4
2.0e44
9.3e-1
3.2e17
1.5e-5
40
1785
8.6e-9
6.1e-4
3.4e-7
2.4e41
1.1e-3
2.0e15
1.5e-7
40
1785
8.4e-11
6.0e-6
3.4e-9
2.4e39
1.1e-5
Notes. The δ3max and td values for H2 were assumed to be identical as
for CO. The second column summarises the values extracted from the
best-fit model (first method, see text).
determined by Bourke et al. (1997) based on CO (1−0) and
(2−1) line observations (combined with 13 CO and C18 O (1–0)).
From a different perspective, the value that we found is simultaneously ∼50 times smaller than that found in the bright CO positions of the W44 SNR studied in the same way by Anderl et al.
(2014), where molecular shocks also propagate. The corresponding momentum is 0.4 M km s−1 , also a good factor ∼100 below
the values inferred using similar methods in W44. This is also
a factor 10 below the value found based on more observational
methods (described in the next paragraph) over a 12.00 5 beam in
the massive star-forming region W28 A2 that encompasses three
different outflows at least (Gusdorf et al. 2015). Finally, the dissipated energy is 4.2 × 1043 erg, typically two orders of magnitudes below the equivalent quantity in W44. This is also two
orders of magnitudes below the energy dissipated in a 12.00 5 beam
in W28 A2 but comparable to what was found by Gomez-Ruiz
et al. (2013) for the entire outflows (associated with low-mass
YSOs) L1448-IRS3 and HH211-mm (based on the use of more
observational methods). From this method, it seems that BHR71
can be defined as an energetic low-mass outflow, but the relatively high numbers we found could be the effect of the method
we used, based on a sophisticated shock model. From the energetic point of view, the impact of the whole BHR71 outflow on
the Galactic ISM is much less than that, e.g. of the whole W44
SNR, because the dissipated energy is smaller, but also because
of its much smaller size: the BHR71 outflow is roughly covered
by a rectangle of ∼0.1 × 0.5 pc2 , whereas a SNR such as W44
resembles a circle of ∼26 pc radius.
We also used a second method, following Bontemps et al.
(1996), Beuther et al. (2002), to calculate the outflow parameters related to the considered species (CO, H2 and SiO). These
˙
parameters (dynamical age td , mass M, mass entrainment rate M,
momentum P, mechanical force Fm , kinetic energy Ek , and mechanical luminosity Lmech ), are calculated using the equations:
td = R/δ3max ,
(1)
M = N × πR × mspecies ,
˙ = M/td ,
M
P = M × δ3max ,
Fm = M × δ3max /td ,
(2)
(3)
(4)
(5)
Ek = M × δ32max /2,
Lmech = Ek /td ,
(6)
(7)
2
where N is the column density of the considered species, R the
radius of our analysis region, and 3max the approximate zerobase linewidth of the considered species. We used CO, H2 , and
A98, page 11 of 14
A&A 575, A98 (2015)
SiO column densities extracted from our best-fit model associated with a filling factor of 1. The results are indicated in Table 3.
Lacking the necessary spectral resolution for the H2 data, we assumed that the δ3max and td values for H2 are identical identical as for CO. This leads to overestimating the mass, momentum, and energy calculated based on the H2 data with respect
to the results purely obtained from the model (first method).
This is because the global shock velocity of our best-fit model,
22 km s−1 , is less than the zero base linewidth that we attributed
to H2 in this second method. Regardless, the CO mechanical luminosity we constrain is again consistent with that measured by
Bourke et al. (1997) for the whole outflow (0.5 L ), whereas the
corresponding value we calculated for H2 exceeds that given by
N09 for the fraction of the outflow that they mapped (0.05 L ).
We can now compare the outflow parameters as inferred
from our observations of BHR71 with similar studies found in
the literature for outflows associated with protostars of various masses. An impressive number of such studies have been
aimed at isolated targets, making it difficult to give some perspective on our results based on a one-to-one comparison (e.g.
Leurini et al. 2006; Lebrón et al. 2006; Fontani et al. 2009; Liu
et al. 2010; Guzmán et al. 2011; Cyganowski et al. 2011). We
consequently focus on a comparison with “survey studies”, i.e.
studies that are aimed at extracting outflow parameters from a
sample of sources. From this respect, we found that the outflow parameters derived from CO observations of BHR71 are
obviously systematically less than the values inferred in more
massive environments (e.g. Duarte-Cabral et al. 2013; Klaassen
et al. 2011), let alone in the outflows associated with O-type
stars (López-Sepulcre et al. 2009). This is also true for SiOrelated energetic parameters: BHR71 is less energetic from this
point of view than its massive counterparts (Sánchez-Monge
et al. 2013). Indeed these authors computed the mass, momentum, and energy associated with the SiO (2−1) and (5−4)
lines. Their study was focussed on the whole outflows, resulting in values considerably larger than those inferred here (4-110
and 2−35 M ; 26−2130 and 11−440 M km s−1 ; (0.2−75) and
(0.06−16) × 1046 ergs). On the contrary, the energetic parameters inferred from our study of one bright beam are similar to
those inferred over the extent of the whole outflows associated
with similar low-mass stars as IRS1, for instance. This is the
case for the outflows in the Perseus cloud (NGC 1333, L1448,
HH211, L1455, e.g. Curtis et al. 2010; Gomez-Ruiz et al. 2013).
The conclusion is that either BHR71 is indeed more energetic
than its low-mass counterparts, or that our method partly based
on shock modelling naturally yields higher numbers than in all
these studies, that often make use of fewer CO lines, for instance,
than in the present work.
by the fact that we had to assume different filling factor values
for the different considered species. From the analysis of our
ten best-fit models, we consider that the constrained values are
quite robust, as these models all had nH = 104 cm−3 , b = 1−2,
3s = 20−25 km s−1 , and an age of 3500−4500 years.
However this modelling can still be used to discuss the feedbacks of the shocks encompassed in our observations. We studied its chemical feedback in terms of SiO chemistry, placing an
upper limit of 4% of the total silicon-bearing material in the form
of SiO in the grain mantles in the pre-shock region.
We quantified the contribution of shocks to the excitation
of CO around low-mass protostars surrounded by outflows, and
shown that the CO excitation diagram from a shocked layer
where the gas temperature is calculated point by point can be
interpreted as a two-component one for levels from Jup = 12
to 40.
We inferred global outflow parameters from our shock
model: a mass of 1.8 × 10−2 M was measured in our beam,
in which a momentum of 0.4 M km s−1 was dissipated, corresponding to an energy of 4.2 × 1043 erg. We also analysed the
energetics of the outflow species by species. Both methods suggest that BHR71 is a rather energetic outflow.
Three perspectives lie ahead of the present study. The first
is to generalise our analysis to the whole outflow, and to observationally constrain the outflow energetic parameters over the
whole mapped area. This will yield meaningful comparisons
with observational studies of various similar objects. The second is to observe the SiO knot position with ALMA to resolve
the multiple shock structures caught in our present single-dish
beam. This should allow us to isolate a single shock structure,
which would help us to get closer to the geometrical configuration that the Paris-Durham model was tailored to fit, and lead the
shock analysis to a new level of understanding, both in terms of
chemistry and energetics. Finally, at this point, more emission
lines will also have to be included in our analysis, especially
H2 O lines.
Acknowledgements. We thank the anonymous referee for comments that helped
to improve the quality of this work. We thank the SOFIA operations and the
GREAT instrument teams, whose support has been essential for the GREAT
accomplishments, and the DSI telescope engineering team. Based [in part] on
observations made with the NASA/DLR Stratospheric Observatory for Infrared
Astronomy. SOFIA Science Mission Operations are conducted jointly by the
Universities Space Research Association, Inc., under NASA contract NAS297001, and the Deutsches SOFIA Institut, under DLR contract 50 OK 0901.
This work was partly funded by grant ANR-09- BLAN-0231-01 from the French
Agence Nationale de la Recherche as part of the SCHISM project. It was also
partly supported by the CNRS programme “Physique et Chimie du Milieu
Interstellaire”.
Appendix A: Filling factors
6. Conclusions
We have presented new observations of the BHR71 bipolar outflows, obtained with SOFIA in 12 CO (11−10), and (16–15), and
with APEX in 12 CO (3−2), (4–3), (6–5), (7–6), and 13 CO (3−2).
We combined these data with existing datasets: in H2
(Spitzer/IRS) and SiO (APEX), and we have compared the observations in the form of a CO flux diagram, an H2 excitation
diagram, and an SiO integrated intensity diagram to a grid of
sophisticated shock models.
Our best fit is non-stationary (CJ-type) and has the following
parameters: nH = 104 cm−3 , b = 1.5, 3s = 22 km s−1 , and an age
of 3800 years. The age and velocity of the shock model hint at
the presence of more than one shock structure within the rather
large beam of our observations, 2400 . This was also suggested
A98, page 12 of 14
The half-maximum emission contours of the CO (6−5) and
(3−2) lines were already shown in Fig. 2, in thick black and red
contours, at their nominal resolutions of 900 and 18.00 1. For both
the SiO peak and knot, the filling factor of the emission with
respect to a beam of 2400 (that of the CO (11−10) line observations), inferred from the red, CO (3−2) contours is of the order
of 1. As we performed our analysis over a 2400 beam size, and
not at the resolution of the CO (3−2) line, we decided to verify
that this filling factor assumption was correct for all CO transitions at a 2400 resolution. We hence convolved all the APEX data
to this resolution. The result is shown in Fig. A.1. The dataset
is remarkably consistent in terms of the size of the emitting region: the half-maximum emission contour for all lines is broadly
the same, except for the (7–6) transition, which could be due
A. Gusdorf et al.: Impacts of pure shocks in the BHR71 bipolar outflow
Appendix C: The CO flux diagram in the SiO peak
position
Fig. A.1. Same field as in Fig. 2, shown in CO (6−5) (colours) convolved at the 2400 resolution. The half-maximum contours of the emission of the CO (3−2) (black line), (4–3) (red), (6–5) (green), and (7–6)
(blue) lines is also shown at the same 2400 resolution. The SiO peak and
knot, HH320 AB and knot 5 positions, as well as the beam sizes of the
(6–5), (16–15) and (11–10) transitions are also indicated as in Fig. 2.
to insufficient signal-to-noise values. Based on this figure, we
chose to operate with a filling factor of 1 for all CO transitions
observed in the SiO knot position over a beam of 2400 .
Appendix B: CO, SiO, and H2 observables
Table B.1. CO and SiO integrated intensity values, and H2 level populations measured over a beam of 2400 centred on the SiO knot position.
Observable
R
T MB d3 (K km s−1 )
R
T MB d3 (K km s−1 )
Observable
ln (N/g)
Species
Line
SiO knot
CO
CO
CO
CO
CO
CO
SiO
SiO
SiO
Species
H2
H2
H2
H2
H2
H2
H2
H2
(3–2)
(4–3)
(6–5)
(7–6)
(11–10)
(16–15)
(5–4)
(6–5)
(8–7)
Level
(0, 0)
(0, 1)
(0, 2)
(0, 3)
(0, 4)
(0, 5)
(0, 6)
(0, 7)
113.4 ± 17.0
117.3 ± 17.6
115.2 ± 17.3
95.2 ± 14.3
22.9 ± 3.4
3.4 ± 0.5
10.5 ± 1.1
5.6 ± 0.6
2.5 ± 0.3
SiO knot
43.5 ± 1.2
41.0 ± 1.3
40.3 ± 1.3
38.9 ± 1.4
38.2 ± 1.4
36.7 ± 1.4
35.7 ± 1.4
34.6 ± 1.5
Notes. This means that the CO (16−15) (nominal resolution of 16.00 3)
and SiO (nominal resolution of 2800 ) data were a posteriori corrected
assuming the emission is extended over the largest beam. As such, the
initial data were multiplied by (16.3/24)2 and (28/24)2 . The uncertainties are given. The integrated intensities were calculated between −4.5
and 50 km s−1 . The CO and SiO integrated intensities correspond to a
filling factor of 1.
Fig. C.1. Observational CO flux diagram over a beam of 2400 obtained
in the SiO peak position. The CO (16−15) line is not detected: an upper
limit estimate is displayed in the form of a black arrow.
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